In piping engineering, various methods are known to radially expand a pipe. For example, there are a method of injecting water into a divided longitudinal range at an end of the pipe and expanding the diameter of the pipe using hydraulic pressure, and a method of inserting a roller from one end of the pipe and mechanically expanding the diameter of the pipe using a pressing force from the roller, which are only for expanding the end of the pipe.
Also, NPL 1 mentioned below has an object to relieve residual stress at a weld joint. Meanwhile, NPL 1 discloses a method of forming ice plugs at two longitudinal spots in a pipe with a weld joint therebetween, and plastically deforming the pipe using volume expansion and an increase in internal pressure when water trapped between the two ice plugs turns into ice as the ice plugs grow.